Another big demonstration in Paris overnight by opponents of same-sex marriage. Organisers claimed there were 1.4 million of them, although the police estimate was a more believable 300,000. Still, that’s a lot of people. While the main protest was apparently peaceful, there was violence on its fringes as police used batons and tear gas against demonstrators who had crossed barricades to occupy the Champs-Elysées.

This is pretty much the last throw for the French anti-same-sex marriage movement. The legislation they’re fighting has already been passed by the National Assembly, and will be voted on by the Senate next month – where the government’s comfortable majority should ensure passage. And once passed, it’s almost unthinkable that a future government would repeal it.

As I said last month, “Opposition to same-sex marriage is therefore fed by the knowledge that it has to be stopped in the beginning if it’s to be stopped at all.”

(Those who read French will enjoy Le Monde’s comprehensive investigation into the bona fides of the anti-same-sex marriage coalition, “Manif pour tous” – “demo for all”, a play on “marriage for all”. Not surprisingly, they found a large number of front groups controlled by religious fundamentalists.)

Meanwhile the issue is on the boil this week in the United States as well, as the Supreme Court prepares to hear argument in the case of Hollingsworth v. Perry, the challenge to the voter-initiated amendment to California’s constitution in 2008 that purported to ban same-sex marriage.

The district court that initially heard the case (then called Perry v. Schwarzenegger) held that the amendment was invalid because it breached the fourteenth amendment’s guarantee of equal protection of the laws – thus implicitly finding that there was a federal constitutional right to same-sex marriage. On appeal, the Ninth Circuit appeal court upheld the ruling but narrowed its basis, arguing that the Californian move was invalid because it took away an established right, and that therefore it was not necessary to decide the broader question.

Opponents of same-sex marriage are hoping the Supreme Court will overturn both decisions and reinstate the California ban; their fear is that it will go the whole way and legalise same-sex marriage across the nation. But the most likely result is that it will do neither, but will uphold the appeal court’s decision on grounds that confine the impact to California.

Because the basic argument against the ban on same-sex marriage is that it targets a particular group of people for no rational reason, its supporters have been trying to come up with a justification that doesn’t seem to just boil down to dislike of gay people. Last week, Ashley Parker in the New York Times profiled the young activists of what they call the “pro-marriage movement” engaged in just such a search; as one said, “To the extent that the other side is able to frame this as a vote for gay people to be happy, it will be challenging for us.”

Most supporters of same-sex marriage would say that the activists are just trying to find a rationalisation for bigotry and/or fundamentalist religious belief. Jon Chait remarks that they “are pretty much all religious Christians who oppose gay marriage because their church does, and they need to reverse engineer a public policy rationale.”

No doubt that’s largely true, but I think there’s more going on here. Opponents of same-sex marriage have always argued that it represents a threat to “traditional” marriage, but have resisted explaining just what they mean by that. Now that just beating up on gays for its own sake has become unfashionable, they’re starting to come clean.

Hence one of the activists quoted by the Times says “It’s really a broader defense of marriage and a stronger marriage culture,” while another cites the protection of “monogamy, sexual exclusivity and permanency.”

That’s still window-dressing, but it’s getting closer to the truth. What they really want to defend is the idea of marriage as a hierarchical institution.

If your picture of heterosexual marriage is one where it basically serves the husband’s interests, where he is in charge and wife and children are his subordinates, and where controlling the wife’s sexual behavior is fundamental to it in a way that controlling the husband’s is not – and this is still the ruling picture in much of America (not to mention the rest of the world) – then same-sex marriage really is a threat, because it shows that marriage need not be a union of unequals and can be non-hierarchical.

It seems to me that that’s the issue at the heart of the same-sex marriage battle: it’s not just fear of the gays (although of course that’s deeply entwined with it), it’s fear of losing control within heterosexual marriage.